1. Field of the Invention
The present invention generally relates to photoelectric transfer or transducer devices utilized in an image inputting device such as a digital (color) copying machine, an image data inputting device such as a desk top publishing system, a reading device for original documents such as a facsimile machine, or an image pickup device such as a VDT (Visual Display Terminal).
2. Description of the Prior Art
A photoelectric transfer device is known in which a depletion type N-channel MOS field effect transistor (MOSFET) is used as an amplifier element. Generally, such a photoelectric transfer device uses a plurality of photoelectric transfer elements arranged in a line or in rows and columns and which outputs electric signals based on the quantities of incident light in a time sequence.
FIG. 1 shows a conventional photoelectric transfer device as described above. The conventional photoelectric transfer device includes a plurality of (n: an integer) photoelectric transfer elements arranged in a line or in rows and columns, and a plurality of (n) photoelectric transfer cells associated with the n photoelectric transfer elements. Since the n photoelectric transfer cells have the same structures as each other, only one photoelectric transfer cell (corresponding to one bit) is illustrated for the sake of simplicity.
The photoelectric transfer cell shown in FIG. 1 is made up of a photoelectric transfer element PD formed with a photodiode, an amplifier element Q formed with a depletion type N-Channel MOSFET, a first initializing means (a switch element) S1, a second initializing means (a switch element) S2 and a read means (a switch element) S3. The first initializing means S1 performs a first initializing operation in which the gate of the amplifier element Q is set to a potential VR1. The second initializing means S2 performs a second initializing operation in which the source of the amplifier element Q is set to a potential VR2. The read means S3 outputs, as an output signal from the photoelectric transfer cell, the source potential of the amplifier element Q to a common signal line CM. One terminal of the photoelectric transfer element PD is connected to the first initializing means S1 and the gate of the amplifier element Q, and the other terminal of the element PD is maintained at a voltage Vcc applied to the drain of the amplifier element Q. The output signal output to the common signal line CM is amplified by an amplifier AMP of the final stage, which outputs an amplified output signal as the final output signal.
At the commencement of the operation of the photoelectric transfer device shown in FIG. 1, the first initializing means S1 and the second initializing means S2 are switched to the closed state, and hence the gate and source of the amplifier element Q are set to the potentials VR1 and VR2, respectively. After the initializing operation of the first and second initializing means S1 and S2 is completed, the source potential of the amplifier element Q is settled at a potential which is higher than the gate potential thereof by a gate-source voltage Vth thereof. In this state, when a photoelectric current dependent on the quantity of incident light flows in the photoelectric transfer element PD, the photoelectric current charges a parasitic capacitor of the first initializing means S1, a parasitic capacitor of the second initializing means S2, and a parasitic capacitor of the photoelectric transfer element PD. That is, a capacitor (storage capacitor) coupled to the gate of the amplifier element Q is charged. When charging of the capacitor (storage capacitor) connected to the gate of the amplifier element Q is completed, the gate potential of the amplifier element Q is raised. A source current (drain current) dependent on the above increase in the gate potential flows in the amplifier element Q. This source current charges a stray capacitor and the parasitic capacitor of the second initializing means S2, so that the source potential of the amplifier element Q is raised as much as the increased gate potential. That is, the amplifier element Q functions as a source follower, and the source potential of the amplifier element Q changes so as to follow up to the gate potential. The gate potential depends on the magnitude of the photoelectric current, and hence the source potential reflects the magnitude of the photoelectric current. Thus, the source potential can be read as the result of the photoelectric transferring operation of one photoelectric transfer cell, that is, as the output signal.
Factors determining the sensitivity of the above-mentioned photoelectric transfer device are the sensitivity (the ratio of the photoelectric current to the amount of the incident light) of the photoelectric transfer element PD, and the capacitor (storage capacitor) coupled to the gate of the amplifier element Q. In order to enhance the sensitivity, it is necessary to improve the sensitivity of the photoelectric transfer element PD or decrease the storage capacitance. More concretely, normally, the quantum efficiency of the photoelectric transfer element PD is as high as 90% or higher, and the photoelectric current is approximately based on the dimensions of the photoelectric transfer element PD. Hence, in order to improve sensitivity, it is necessary to decrease the storage capacitance if the size of the photoelectric transfer element PD is not to be diminished.
However, in the photoelectric transfer device shown in FIG. 1, most of the storage capacitance is the parasitic capacitance of the photoelectric transfer element PD. When attempting to improve the sensitivity of the photoelectric transfer element PD, the parasitic capacitance of the photoelectric transfer element PD is increased and hence the storage capacitance is increased. That is, increasing of the sensitivity of the photoelectric transfer element PD is inconsistent with decreasing of the storage capacitance. For the above reason, it is very difficult to increase the sensitivity of the whole device. More particularly, except for CCDs (Charge-Coupled Device), the conventional photoelectric transfer elements PD have large parasitic capacitances, which functions as a storage capacitance. Hence, it is very difficult to reduce the storage capacitance. Further, the CCDs have a large capacitance of a part which converts a charge into a voltage, and do not have high sensitivities. Taking into account the above matter, the structure shown in FIG. 1 is provided with the final-stage amplifier AMP. However, the signal from the amplifier element Q is very small and penetrates through a distance of approximately 10 mm until it reaches the amplifier AMP. Thus, the signal from the amplifier element Q is liable to be affected by either external noise or both or transmission noise. Further, the amplifier AMP introduces noise which cannot be ignored. Even if improvement in sensitivity is attempted by means of the amplifier AMP, it is very difficult to obtain a high S/N ratio.
Furthermore, the photoelectric transfer device of the amplifying type uses a large number of elements within one photoelectric transfer cell. Hence, in an integrated circuit device, in which the elements are integrated on a chip, there is not a high integration density if specific considerations for the layout of the elements is not taken. This prevents application to a sensor using a reduced optical system.